This invention relates to a lightweight impact-resistant window panel, for example for adhesive glazing to the airframe of an unpressurised aircraft.
There is a need for bird-impact resistant window panels for use as the front window panels of unpressurised aircraft, particularly helicopters. Suitable lightweight helicopter window panels have been produced by bonding an outer glass ply to an inner ply of impact-resistant plastics, such as stretched acrylic. The outer glass ply is typically 2 mm to 3 mm thick and the inner plastics ply typically about 4 mm thick. The differences in the coefficients of thermal expansion of the glass and impact-resistant plastics plies lead to thermal stresses as the temperature varies, and such stresses tend to cause shape distortion and deformation of the panel. The problems caused by the stresses are most serious in irregularly shaped flat window panels and curved window panels, especially window panels of varying curvature.
It has been usual to bolt the window panels in position in the airframe, so that substantial forces have been available to align the periphery of the window panel with the frame and secure the panel in position. Despite this, it has sometimes been necessary to specify that the window panel should only be fitted within a narrow specified temperature range.
There is now a requirement for lightweight, impact-resistant window panels for unpressurised aircraft which can be secured in position without the need for bolts, for example by adhesive glazing to the airframe, and can be fitted over a wide range of temperatures.
The invention is based on the discovery that by using a more symmetrical cross-section of the laminated window panel in which two plies of glass are used and are laminated with two thin plies of impact-resistant plastics material, one between the two glass plies and the other on the rearward face of the panel, the problem of mismatch of shape over the range of operating temperatures is overcome and the fit to the airframe is achieved over the range of fitting temperatures.